This invention is generally concerned with alarm circuits for sensing thermal stresses in integrated circuit chips, and is specifically concerned with a circuit formed from an array of semiconductor devices already included within the geography of the chip that exploits the inherent variancy of the conductivity of such semiconductor components to temperature to form the alarm circuit.
Thermal stress has long been recognized as a significant problem in the operation of integrated semiconductor chips. These chips have limited operating temperature ranges, and may not function properly in very hot or very cold environments. In addition, the chips themselves use power and generate heat. Thus, even when the temperature of the environment is within the operating temperature range for the chip, the chip itself may be much hotter. This problem is exacerbated with newer chips that run much faster and thus generate more heat.
Thermal stress may prevent reliable operation of these chips. In many applications of such circuits, such as in the control systems of high-speed aircraft, any such unreliability, no matter how momentary, is unacceptable. Worse yet, thermal stress can destroy the chips. Operating a chip when its substrate is above a certain temperature may damage the miniaturized circuits to the extent that the chip will never operate properly again. However, the applicant has observed that if the chips are not operated while hot, they can survive exposure to much higher temperatures. Thus, the reliability of a particular circuit could be maintained if the operation of certain chips experiencing an overheated condition could be shut off and their work performed by alternative chips. As very large scale integrated circuit chips (such as the VLSICs used in new-generation microprocessors) and custom-designed, special purpose chips may cost hundreds or thousands of dollars each, significant savings could also be realized by preventing the operation of such chips in an overly hot environment.
In view of the foregoing, it would be desirable to have a highly effective and inexpensive means of monitoring chip temperature so that the chip could be turned off, an alarm sounded, etc. before excessively high or low operating temperatures caused the chip to operate unreliably or to become damaged. The environmental temperature of a chip could be monitored using temperature sensors disposed in a computer unit or attached to the chip case itself, but these methods would be expensive in that they require sensors separate from the chip, and additional monitoring hardware. In addition, methods using sensors would provide a less reliable indication of temperature. It is the temperature of the silicon substrate that determines the thermal stress thereon, but the other methods described here sense chip case temperature or the ambient air temperature in a computer unit. The temperature of the chip cases or surrounding air may not accurately reflect the temperature of the circuit substrate.
It is also well-known in the art that the flow of electrons and holes across semiconductor junctions varies with temperature. Generally, in digital electronics, the applicant has observed that the analog property of transistors is considered to be an operational flaw that must be designed around so that the resulting digital circuit is not affected by such temperature variations. Most commonly, the semiconductor chip engineer normally uses transistors in such digital circuits only at either a high or a low voltage (commonly 5 volts and zero volts) so that, regardless of temperature, the transistor will be "on" or "off," respectively.